Method of determining lifetime of electrical and mechanical components

ABSTRACT

The disclosure is directed to systems and methods by which the lifetime, e.g., remaining life or amount of life used, of variable use items, such as rechargeable batteries, battery relays, vehicles and power tools, can be determined that takes into account the conditions of the use of the item. The systems and methods involve an algorithm that can be described as accumulating points based on the real time utilization of the item, e.g., rechargeable battery, battery relay, vehicle, or power tool, and when an agreed-upon number of points have been accumulated, the item can be considered to be at end of life or end of warranty.

TECHNICAL FIELD

The disclosure relates generally to usage of variable use systems, and,in particular, to systems and methods for determining the lifetime ofvariable use systems.

BACKGROUND

Current methods by which the lifetime, e.g., for warranty purposes, ofelectrical and mechanical components typically involve merely keepingtrack of how long the component has been used. For example, batterylifetimes and warranties are generated and agreed upon between thebattery supplier and a vehicle manufacturer depend on methods that areseparate from the actual utilization of battery system. Historical meansof assigning a battery warranty have been based on number of miles/kmdriven or number of days elapsed since the initial vehicle sale. Thismethod does not take into account how aggressively or tamely the batterysystem has been utilized. Further, the farther from a Battery ElectricVehicle that a system architecture is designed (closer toward a mildhybrid) the less correlation there is between vehicle age and mileageand longevity of the battery system. A recent advancement has been toutilize amp-hour throughput as a metric by which to define a warrantyperiod. While this is a closer metric by which to define battery life,it still does not account for the type of utilization (abusiveutilization or mild utilization) of the battery system.

Current methods by which relay warranties as described by a relaymanufacturer is to describe a bin system that indicates the number ofon/off cycles of a relay possible with a certain amount of currentflowing. The higher the current the flows at the moment of relay openingor closing, the lower the lifetime of the relay contacts. There iscurrently no means by which to signal the end of warranty or state ofhealth of the relay when the magnitude of the current at open/close isdifferent every time.

Current methods for determining lifetime for warranty purposes forvehicles and power tools typically comprises merely tracking the numberof miles in the case of vehicles and tracking the how long the tool wasused, e.g., turned on, in the case of power tools. A method ofdetermining the lifetime of vehicles and power tools that takes intoconsideration the conditions of use would be greatly beneficial.

SUMMARY

According to one embodiment, a method of operating a rechargeablebattery of a vehicle includes detecting state of charge swings duringoperation of the rechargeable battery; determining a first point valuefor each of the state of charge swings, the first point value beingdependent on a magnitude of a swing of the respective state of chargeswings; determining a temperature for each of the state of chargeswings; determining a second point value for each of the state of chargeswings, the second point value being dependent upon a magnitude of thetemperature of the respective state of charge swings; determining abattery current for each of the state of charge swings; determining athird point value for each of the state of charge swings, the thirdpoint value being dependent upon a magnitude of the battery current ofthe respective state of charge swings; determining a fourth point valueof each of the state of charge swings, the fourth point value being aproduct of the first point value, the second point value and the thirdpoint value of the respective state of charge swings; determining afirst score for the rechargeable battery, the first score being a sum ofthe fourth point values for the state of charge swings; and correlatingthe first score to a remaining life value for the rechargeable battery.

According to another embodiment, a rechargeable battery system includesa rechargeable battery; a sensor system configured to detect state ofcharge, a temperature, and a battery current of the rechargeablebattery; and a control system configured to receive the state of charge,the temperature, and the battery current from the sensor system. Thecontrol system is configured to: detect state of charge swings duringoperation of the rechargeable battery; determine a first point value foreach of the state of charge swings, the first point value beingdependent on a magnitude of a swing of the respective state of chargeswings; determine the temperature of the rechargeable battery for eachof the state of charge swings; determine a second point value for eachof the state of charge swings, the second point value being dependentupon a magnitude of the temperature of the respective state of chargeswings; determine the battery current of the rechargeable battery foreach of the state of charge swings; determine a third point value foreach of the state of charge swings, the third point value beingdependent upon a magnitude of the battery current for the respectivestate of charge swings; determine a fourth point value of each of thestate of charge swings, the fourth point value being a product of thefirst point value, the second point value and the third point value ofthe respective state of charge swings; determine a first score for therechargeable battery, the first score being a sum of the fourth pointvalues for the state of charge swings; and correlate the first score toa remaining life value for the rechargeable battery.

According to yet another embodiment, a method of determining remaininglife of a battery relay includes detecting a current through a relayduring each cycle that the relay is closed; determining a cycleendurance value for each cycle, the cycle endurance value beingdependent upon the current detected for each respective cycle; assigninga point value to each of the cycles, the point value being a natural logof the cycle endurance value for the respective cycle; and combining thepoint values for each of the cycles to arrive at a life score for therelay.

According to yet another embodiment, a rechargeable battery systemincludes a rechargeable battery including a relay; a sensor systemconfigured to detect a battery current through the at least one relay;and a control system configured to receive the battery current from thesensor system. The control system is configured to: determine thebattery current through the relay during each cycle that the relay isclosed; determine a cycle endurance value for each of the cycles thatthe relay is closed, the cycle endurance value being dependent upon thebattery current through the relay for each of the respective cycles;assign a point value to each of the cycles, the point value being anatural log of the cycle endurance value for the respective cycle;combine the point values for each of the cycles to arrive at a lifescore for the relay.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a battery system in accordance withthe present disclosure.

FIG. 2 is a graph showing the state of charge (SoC) swings during adrive cycle of a battery of a battery system, such as depicted in FIG.1.

FIG. 3 is an exemplary points curve for the SoC swings, such as depictedin FIG. 2.

FIG. 4 is an exemplary points curve for the battery temperature of thebattery system of FIG. 1 during a SoC swing such as depicted in FIG. 2.

FIG. 5 is an exemplary points curve for the battery current of thebattery system of FIG. 1 during a SoC swing such as depicted in FIG. 2.

FIG. 6 is an exemplary points curve for the idle time duration of abattery system such as depicted in FIG. 1.

FIG. 7 is an exemplary points curve for the battery temperature duringan idle time of a battery system, such as depicted in FIG. 1.

FIG. 8 is an exemplary points curve for the battery SoC during an idletime of a battery system, such as depicted in FIG. 1.

FIG. 9 is a binning table for relay contactors according to the priorart.

FIG. 10 shows the binning table of FIG. 9 with the natural log values ofthe cycles.

FIG. 11 is a graph of the natural log cycles vs. the number of cycles ofthe binning tables of FIGS. 9 and 10.

FIG. 12. is a binning table that divides the cycles of the binning tableof FIG. 9 by 1,000,000 to show points per cycle.

FIG. 13 is a graph of the points per cycle of the binning table of FIG.12.

FIG. 14 is a flowchart of a method of determining remaining life of abattery relay.

DESCRIPTION

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the disclosure is therebyintended. It is further understood that the present disclosure includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the disclosure aswould normally occur to a person of ordinary skill in the art to whichthis disclosure pertains.

The disclosure is directed to systems and methods by which the lifetime,e.g., remaining life or amount of life used, of variable use items, suchas rechargeable batteries, battery relays, vehicles and power tools, canbe determined that takes into account the conditions of the use of theitem. The systems and methods involve an algorithm that can be describedas accumulating points based on the real time utilization of the item,e.g., rechargeable battery, battery relay, vehicle, or power tool, andwhen an agreed-upon number of points have been accumulated, the item canbe considered to be at end of life or end of warranty.

In particular, the point system described herein factors in a variety ofusage conditions that are typically already being measured in thesystem. Combining them to generate a point value during use andaccumulating those points until an agreed-upon threshold is attained isan advancement toward defining the end of warranty or end of lifeperiod. The proposed point system better reflects the actual usage ofthe system versus a crude elapsed time, elapsed mileage metrics or thatonly tracks open/close cycles.

An exemplary embodiment of a battery system 10 in accordance with thepresent disclosure is depicted in FIG. 1. The battery system 10 includesa rechargeable battery 12 having a plurality of battery cells 14, 16,18, 20. Four cells 14, 16, 18, 20 are depicted in FIG. 1 although anysuitable number of cells may be used. The battery cells 14, 16, 18, 20are connected in series. The cells 14, 16, 18, 20 may be also becombined into separate modules. Although a single module is depicted inFIG. 1, any suitable number of modules may be implemented. In oneembodiment, the battery 12 is a lithium ion battery, such as are used inelectric and hybrid vehicles. In other embodiments, other types ofbatteries may be used in the system including nickel-metal hydridebatteries and lead-acid batteries.

As depicted in FIG. 1, relays 22, 24 are provided to selectively couplethe battery to one or more circuits which may include a load and acharging circuit (not shown). The relays 22, 24 are opened and closed tocontrol a flow of a charging current to or a flow of a dischargingcurrent from the battery. A first relay 22 is serially connected to apositive electrode of the battery 12, and a second relay 24 is seriallyconnected to a negative electrode of the battery 12.

A battery control system 26 is configured to control the opening andclosing of the relays 22, 24. The battery control system 26 includes acontroller and electronic storage. The controller may be aself-contained, dedicated mini-computer having a central processor unit(CPU). The electronic storage comprises a memory configured to storedata and instructions for access by the controller.

The control system 26 is configured to monitor various battery usageparameters of the battery via a sensor system 28. The sensor system 28is configured to detect various battery usage parameters, such astemperature, voltage, current, and state of charge (SOC) of the battery,using sensors and direct or indirect methods. In particular, the sensorsystem The battery usage parameters may be detected using varioussensors, such as temperature sensors, voltage sensors, current sensors,and the like, as are known in the art. The sensor system 28 detects thebattery usage parameters and transmits them to the battery controlsystem 26.

During operation of the battery system, battery temperature, voltage andcurrent are monitored by the control system. The control system isconfigured to use the battery temperature, voltage and current toestimate the SOC of the battery and/or the battery cells. The SOC may beestimated in any suitable manner as known in the art, such as bymodeling the battery system with an equivalent circuit.

In accordance with the present disclosure, the control system 26 is alsoconfigured to estimate the remaining life and/or life used of thebattery system based on battery usage parameters, such as state ofcharge swing, battery temperature, battery current, time that battery isidle, and state of charge. In particular, values for the battery usageparameters are correlated to predetermined weights and/or multipliers toarrive at a point value, or score, which reflects the actual usage ofthe battery system. The score can be compared to a predeterminedthreshold value for battery life to determine where the battery is at inits life cycle. When the life score exceeds the threshold, it isdetermined that the battery has reached its end of life or end ofwarranty.

The predetermined threshold score or point value for battery life can beany number. It is typically a value that has been agreed upon with thecustomer. For example, it can be agreed with a customer that a batterywill reach end of life/warranty once it accumulates 1,000,000 points—orany other agreed-to number. This number—which can be arbitrary—willfactor into the magnitude of the individual point weights andmultipliers selected for the battery usage parameters utilized todetermine the score.

In one embodiment, the lifetime score is based on state of charge swingsof the battery. An exemplary formula for calculating the lifetime scorebased on state of charge swings is as follows:Σ(S*To*C)where:

-   -   S=the state of charge swing points (based on magnitude of the        SoC swing)    -   To=cell operating temperature during the SoC swing (multiplier)    -   C=current, based on the peak or RMS current during the SoC swing        (multiplier)

The variables S, To and C are weights and/or multipliers having valueswhich are dependent upon their corresponding parameters. A descriptionof how the weights or multipliers used for the battery usage parametersS, To and C is provided below. Each of the variables can be describedgraphically via curves or via tables. Generic curves depict what ishappening in the battery based on physics and historical testing. A veryreliable set of curves/tables may be generated for a particular cellchemistry based on a wide set of tests that also factor in agingtesting.

S, the state of charge swing, is a key determinant in the lifetime of abattery. It can be determined in real time by noting the peaks andtroughs of the SoC during any given segment of a drive cycle and isindicated by the current switching from positive to negative or viceversa. Swings can be miniscule—from hundredths of a percent to severalpercentage points. Each swing must be accounted for and its point valueaccumulated (and appropriately multiplied with the multipliers).

An example curve showing the state of charge during a drive cycle isdepicted in FIG. 2. Swings are indicated with each peak and trough. Inone embodiment, the swing is represented by a percentage change betweenadjacent peaks and troughs. For example, swing 30 in FIG. 2 indicates a10.2% swing from peak A to trough B, and swing 32 in FIG. 2 indicates a15.1% swing from trough B to peak C. Swings can be minuscule asevidenced by swing 33 indicating a 3.7% swing from peak D to trough E.Each swing must be identified and a corresponding point value assignedto it.

FIG. 3 depicts an exemplary curve showing the point values which may beassigned to each swing based on the swing percentage. It is generally asecond order polynomial between SoC swing percentage and the number ofpoints. Note that the number of points on the vertical scale is forexample only. The actual point value would be based on the total pointvalue that defines end of warranty or end of life, and may be derived inany suitable manner.

The variable To may be considered a multiplier by which the point valuefor the SOC swing is multiplied. Battery cells generally have anoperating temperature range defined where they can maximize theirlifetime. Outside of this temperature range, the lifetime of thebatteries is reduced relative to the temperature range. This multiplieris utilized in order to magnify the point value of an SoC swing if theoperating temperature of the battery during this SoC swing is outside ofthe optimal range. In the optimal temperature range the multiplier is 1and therefore not a factor. An example of a graph/table of thismultiplier is depicted in FIG. 4. The vertical scale is arbitrary andthe actual multiplier and shape of the curve is heavily dependent on thechemistry and its optimal temperature range.

The variable C is a multiplier based on the detected current through thebattery. The current can be the peak current during SoC swing or RMS,for example. The magnitude of the current has implication on thelifetime of the battery as the current density at the interface layersof the battery electrodes can be impacted and lead to a fasterdegradation of the microscopic interfaces. Therefore, the magnitude ofthe current acts as a multiplier to the SoC swing to more accuratelyportray the impact to the microscopic layers of the battery pack. In theplot depicted in FIG. 5 there is a basic curve that can be used plussome additional curves that factor in the cell temperature. If theseadditional temperature-based curves are used, the prior metric—theTo—can be eliminated in favor of the current vs. temperature multipliershown below. There would certainly be more temperature curves torepresent either actual temperatures or ranges. Note that the horizontalscale shows “maximum” of the charge/discharge allowed. This accounts forthe degradation of the battery over time and its ability to deliver amaximum current that as compared to what was possible at the beginningof life.

During operation, the control system is configured to determine orcalculate a point value for each SoC swing that is a product of theswing point value, the To multiplier and the C multiplier for therespective SoC swing. The point value for each SoC swing is then summedwith the point values for each previous SoC swing in order to maintain arunning score that represents the battery usage represented by the SoC.

The control system may also be configured to take idle time intoconsideration in determining the overall lifetime score for the battery.For example, a first score may be derived from the battery usageindicated by the state of charge. A second score may be derived from theidle time conditions, such as duration of idle time, battery temperatureduring idle times and SoC during idle times. An exemplary formula forgenerating the overall battery lifetime score is as follows:Σ(S*To*C)+Σ(d*Ti*H)where:

-   -   S=the state of charge swing points (based on magnitude of the        SoC swing)    -   To=cell operating temperature during the SoC swing (multiplier)    -   C=current, based on the peak or RMS current during the SoC swing        (multiplier)    -   d=duration (hours or days) of key-off idle time. This is a proxy        for calendric aging    -   Ti=cell temperature during the idle time (multiplier)    -   H=SoC during the idle time (multiplier)

The variables S, To and C may be determined as described above. Adescription of how the weights or multipliers used for the battery usageparameters d, Ti and H is provided below. As noted above, each of thevariables can be described graphically via curves or via tables. Genericcurves depict what is happening in the battery based on physics andhistorical testing. A very reliable set of curves/tables may begenerated for a particular cell chemistry based on a wide set of teststhat also factor in aging testing.

The variable d (duration) accounts for the calendaric aging of thebattery based purely on the passage of time. Idle time duration refersto the amount of time that passes while the battery is in an off state,e.g., the vehicle key is in the off position. The key-off state of thevehicle can be monitored by the vehicle control system (ECU 40, FIG. 1).While on the whole this is a small factor relative to the impact ofactual battery usage, it does factor in when the vehicle is not inuse—particularly at high temperatures where self discharge is a factor.An exemplary points curve is depicted in FIG. 6. The idle time can betracked in days, hours or other duration metric. This is generally alinear function.

The variable Ti is a multiplier based on the battery temperature duringthe idle times. An exemplary points curve for the idle time batterytemperature is depicted in FIG. 7. During the battery idle time,chemical reactions continue to occur and are accelerated at highertemperatures based on the Arrhenius equation. This multiplier representsthe faster calendric aging that occurs during idle time.

H is a multiplier based on the SoC of the cells. An exemplary pointscurve for the variable H is depicted in FIG. 8. Cells stored at high SoCdegrade at a faster rate due to the high voltage potential across theelectrodes and the electrolyte. This multiplier accounts for thisscenario to increase the point value of the idle time when the cell SoCis high and degradation occurs at a faster rate. Unless otherwise noted,the point values used in the point curves are arbitrarily selected torepresent the actual conditions faced by the battery during idle times,and may be derived in any suitable manner.

Idle times for the vehicle/battery are identified and point values forthe idle time duration are determined by the control system, e.g. byusing the point curve for the variable d depicted in FIG. 6. Thetemperature multiplier Ti and SoC multiplier H for each identified idletime are also determined, e.g., by using the point curves depicted inFIGS. 7 and 8, respectively. The control system is configured todetermine or calculate a point value for each idle time that is aproduct of the duration, the Ti multiplier and the H multiplier for therespective idle time. The point value for each idle time is then summedwith the point values for each previous SoC swing in order to maintain arunning score that represents the battery conditions during idle timesof the battery.

The running score representing battery usage conditions (i.e., the firstscore mentioned above) and the running score representing idle timeconditions (i.e., the second score represented above) are combined bythe control system to arrive at an overall lifetime score for thebattery. The running scores may be stored in a dedicated location inmemory and updated for each SoC swing and idle time of the battery.

The overall lifetime score for the battery may then be used to determinethe remaining life and/or the amount used. For example, the controlsystem may be configured to compare the overall lifetime score to apredetermined lifetime threshold score. The percentage differencebetween the overall score and the threshold score may then be correlatedto the amount of life remaining in the battery. The predeterminedthreshold score may be selected in any suitable manner and is typicallyused to indicate when the expected life of the battery and/or warrantyof the battery is at an end.

The previous example described an embodiment in which battery usage andidle time conditions are used to generate a lifetime score for thebattery of the battery system. A similar methodology may be used togenerate a lifetime score for other components of the battery system,such as the relays. In one embodiment, the point system accumulatespoints based on the current flow at the moment a relay is opened/closed.The higher the current flow the higher the point value ascribed to thatevent. When a predetermined number of points have been accumulated itmay be considered the end of the warranty or end of life for the relay.

Relay manufacturers typically provide a binning table that describestheir expected lifetime of a relay based on the number of cyclespossible at a given current flow. FIG. 9 depicts an example of such atable. From this table it can be seen that if the contactor isopened/closed into a low current flow of 0.5 A or below, the lifetimecan be expected to be 100,000 cycles. However, the vehicle manufacturerindicates that the currents may be spiking during such event due toconditions external to the battery pack. There is no way to consider theguaranteed lifetime if—for example—there are occasional events thatoccur at higher currents.

By using a point system, the aforementioned issue can be resolved. Byconverting the cycle endurance value into its own natural log value orinto a base 10 log value, it can be seen that the resultant curve has avery high correlation to a second order polynomial and is therefore verypredictable. This means that for a given current at the open/close of arelay, a point value can be assigned and accumulated. A binning tableshowing the natural log value of the contactor cycle endurance values(See, FIG. 9) is depicted in FIG. 10. A graph of the natural log ofcycles (Y-axis) vs. the number of cycles is depicted in FIG. 11.

In a particular case, assuming that 1,000,000 points are to be thedemarcation at which the contactor warranty or lifetime ends, a tablecan be generated that divides the previous table's cycle count into1,000,000 points. Such a table is depicted in FIG. 12. From here,plotting the points table results in a highly correlated 2nd orderpolynomial, as depicted in FIG. 13, where current is on the horizontalaxis and points assigned for opening/closing the contactor at thecurrent is the vertical axis. Using the resultant polynomial curve, if avehicle commands the battery pack to open the contactor when there are320 A flowing, the point value for that event would be 1733 points.Using this aforementioned methodology and continually adding the pointsto a running tally provides a more accurate and reliable reporting ofthe contactor life in a vehicle.

FIG. 14 depicts an embodiment of a method of determining remaining lifeof a battery relay based on the point system described above. Accordingto the method, a current through a relay is detected during each cyclethe relay is closed (block 100). A cycle endurance value is thendetermined for each of the cycles during which the relay is closed(block 102). As discussed above, the cycle endurance value is dependentupon the current detected for each a respective cycle. A point value isassigned to each of the cycles (block 104). The point value is a naturallog of the cycle endurance value for the respective cycle. The pointvalues for each of the cycles are then combined to arrive at a lifescore for the relay (block 106). The life score for the relay is thencompared with a predetermined threshold value to determine the liferemaining for the relay (block 108).

Another instance that may benefit from the points system describedherein is determining the lifetime of a vehicle, and in particular, thevehicle engine. The engine life has typically been determined based onmileage alone. The present disclosure proposes a system that takes intoaccount coolant temperature, engine rotations per minute (RPMs), engineload (i.e., torque), and oil life. The vehicle includes sensors fordetecting coolant temperature, RPMs, engine torque and oil life. Acontrol system for the vehicle receives the values continuously asinputs from the sensor system and can process the inputs to maintain arunning score. In one implementation the multiplier of each of thosemetrics would result in—for example—a number of points per second ofusage of the vehicle.

Coolant score for example can be a non-factor (e.g., a multiplier of 1)when the coolant temperature is in a normal operating range. If thecoolant temperature exceeds the normal operating range, the score forthe coolant temperature can increase to reflect the toll such conditionscan have on the engine. Similarly, for the RPMs, engine torque and oillife, when the variable is within a normal operating range or belowthreshold(s), the weight/score for the variable will be lower than whenthe variable is outside the normal operating range or above a thresholdvalue. The exact weight/score and the corresponding threshold values canbe arbitrarily selected so as to reflect the amount of life that is usedbased on the actual usage of the vehicle. For example, at coldtemperatures, a driver who on a cold start floors the accelerator whiletowing would incur a large number of points because such conditionresults in rapid engine wear. At mild temperatures, a driver who doesnot drive aggressively would incur few points because all conditionshave a low multiplier.

Another system that could benefit from the use of the points trackingsystem described herein is power tool systems. The life of a power toolis often based on the amount of time used, e.g., hours of usage. Thepresent disclosure proposes a system that takes into account motortemperature, motor torque, current flow through the windings of themotor, and motor RPM. The power tool includes sensors for detectingthese parameters. A control system for the power tool receives thevalues continuously as inputs from the sensor system and can process theinputs to maintain a running score for the power tool or for the motorof the power tool. The exact weight/score and corresponding thresholdvalues can be arbitrarily selected so as to reflect the amount of lifethat is used based on the actual usage of the tool. In oneimplementation the multiplier of these metrics would result in—forexample—a number of points per 1/10th of a second of usage. For example,if a user stalls the motor during a drilling or screwing operation andthe current spikes and the winding temperature rises rapidly, therewould be a high number of points based on the duration that the userkeeps the trigger depressed. If a user is drilling into soft wood andthe motor isn't strained, there would be very few points accumulatedduring such an operation

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe disclosure are desired to be protected.

What is claimed is:
 1. A method of determining remaining life of abattery relay, the method comprising: detecting a current through arelay during each cycle that the relay is closed; determining a cycleendurance value for each of the cycles during which the relay is closed,the cycle endurance value being dependent upon the current detected fora respective cycle; assigning a point value to each of the cycles, thepoint value being a natural log of the cycle endurance value for therespective cycle; and combining the point values for each of the cyclesto arrive at a life score for the relay.
 2. The method of claim 1,further comprising: comparing the life score for the relay to apredetermined threshold value; and determining a life remaining for therelay based on the comparison.
 3. A rechargeable battery systemcomprising: a rechargeable battery including a relay; a sensor systemconfigured to detect a battery current through the relay; and a controlsystem configured to receive the battery current from the sensor system,the control system being configured to: determine the battery currentthrough the relay during each cycle that the relay is closed; determinea cycle endurance value for each of the cycles during which the relay isclosed, the cycle endurance value being dependent upon the batterycurrent through the relay for a respective cycle; assign a point valueto each of the cycles, the point value being a natural log of the cycleendurance value for the respective cycle; and combine the point valuesfor each of the cycles to arrive at a life score for the relay.
 4. Thesystem of claim of claim 3, wherein the control system is furtherconfigured to: compare the life score for the relay to a predeterminedthreshold value; and determine a life remaining for the relay based onthe comparison.